CI: Properly disable Python in the 'nobindings' CI job

Python was explicitly enabled, presumably a copy'n'paste error.

.gitlab-ci.yml

@@ -20,7 +20,7 @@ ubuntu-20.04-clang-10-20-nobindings:
   stage: test
   variables:
     DUNECI_TOOLCHAIN: clang-10-20
-    DUNECI_CMAKE_FLAGS: '-DDUNE_ENABLE_PYTHONBINDINGS=ON'
+    DUNECI_CMAKE_FLAGS: '-DDUNE_ENABLE_PYTHONBINDINGS=OFF'
   tags: [duneci]
 
 # Check for spelling mistakes in text